Flexible circuit board

ABSTRACT

In a flexible circuit board, signal wirings and ground wirings are provided on one main surface of a base film formed of a thermoset resin. A coverlay film formed of a thermoplastic resin is adhered to and integrated with the signal wirings, ground wirings, and base film. External terminals  15  are disposed in a predetermined conductor pattern on one main surface of the coverlay film, and a plated layer is formed on each of the external terminals. A first ground layer and a rear side resin film are adhered in this order below the base film to be integrated.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims the benefits of Japanese Patent Application No.2010-266989 filed Nov. 30, 2010 which is hereby incorporated byreference herein its entirely.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flexible circuit board on which atransmission wiring, a circuit wiring, and the like are arranged and,more particularly, to a flexible circuit board using a dielectric layerformed of a thermoplastic resin between the wirings, having excellenthigh-frequency signal transmission characteristics, and facilitatingachievement of a multilayer configuration.

2. Description of the Related Art

A flexible circuit board used for an electronic device such as a networkdevice, a server, and a tester, or for cable connection is required totransmit a high-speed digital signal having a band of several GHz toseveral tens of GHz without losing high-frequency characteristics.Further, in a mobile electronic device like a portable device,high-density wiring and short, small, light, and thin design is nowproceeding under a design trend of short, small, light, and thin of suchan electronic device. Under such circumstances, miniaturization of aconductor pattern serving as a transmission wiring or circuit wiring andreduction in the space between the conductor patterns are proceeding,and further, implementation of a multilayer structure of the conductorpattern is now underway.

A liquid crystal polymer which is a thermoplastic resin is disclosed asa preferred example of a dielectric layer to be formed between theconductor patterns in a flexible circuit board (refer to, e.g., Jpn.Pat. Appln. Laid-Open Publication No. 2001-135974). This thermoplasticliquid crystal polymer is low in its dielectric constant and dielectrictangent and extremely low in water and moisture absorbency. Therefore,excellent transmission characteristics of a high-frequency signal in theflexible circuit board, that is, excellent transmission/conductioncharacteristics based on stably low reactance, impedance, andtransmission loss can be obtained.

However, in the case where two or more flexible circuit boards aremultilayered by laminating interlayer dielectric layers formed of thethermoplastic liquid crystal polymer and applying heating andpressurization (hereinafter, referred to also as “hot press”) to theresultant structure, a position gap of the conductor patterns or aconductive member connecting the conductor patterns easily occurs and,at the same time, the positioning accuracy in the laminated structure isdeteriorated to easily cause a failure in electrical connection betweenlayers, which makes miniaturization of the conductor pattern and thelike difficult.

Generally, in the multilayering process of the wiring board conducted bythe hot press, the thermoplastic resin serving as the interlayerdielectric layer is soften, which makes it very difficult to adopt alamination method of laying up a plurality of layers on a layer-to-layerbasis. Further, even in a method of laminating a required number ofthermoplastic resin films each on which the conductor pattern and thelike are formed and integrated by one hot press process (refer to, e.g.,Jpn. Pat. Appln. Laid-Open Publication No. 2009-302343), the positiongap of the conductor pattern in the connection portion due to softeningor melting of the laminated thermoplastic resin films inevitably occurs.Thus, high-level technical management and high-performance manufacturingfacility are essential for the manufacturing process.

As described above, in the multilayering process of the flexible circuitboard achieved by laminating a plurality of thermoplastic resin films bythe hot press, high-level production management and quality managementare required to make it difficult to achieve a reduction in themanufacturing cost, resulting in difficulty in cost reduction of theflexible circuit board.

The present invention has been made to solve the above problems, and anobject thereof is to provide a flexible circuit board having aninterlayer dielectric layer formed of a thermoplastic resin andexcellent in high-frequency signal transmission characteristics, andfacilitating achievement of a multilayer configuration. Another objectof the present invention is to provide a flexible circuit board capableof simplifying a manufacturing method of a high-performance flexiblecircuit board and reducing the cost thereof.

SUMMARY OF THE INVENTION

To achieve the above object, according to an aspect of the presentinvention, there is provided a flexible circuit board including: a firstinsulating layer formed of a thermoplastic resin having a conductivelayer on its one main surface; and a second insulating layer formed of athermoset resin having a conductive layer on its one main surface andadhered to and integrated with the first insulating layer. The curingtemperature of the thermoset resin is lower than the glass-transitionpoint or melting point of the thermoplastic resin, and the elasticmodulus of the second insulating layer formed of the thermoset resin islower than the elastic modulus of the first insulating layer formed ofthe thermoplastic resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially enlarged perspective view illustrating an exampleof a flexible circuit board according to a first embodiment of thepresent invention;

FIG. 2 is a partially enlarged perspective view illustrating anotherexample of the flexible circuit board according to the first embodimentof the present invention;

FIGS. 3A to 3C are views illustrating the flexible circuit boardaccording to the first embodiment of the present invention, in whichFIG. 3A is a cross-sectional view taken in the X direction of FIG. 1,FIG. 3B is a cross-sectional view taken along Y direction of FIG. 1, andFIG. 3C is a cross-sectional view taken along Z direction of FIG. 2;

FIGS. 4A to 4E are manufacturing process cross-sectional viewsillustrating an example of a manufacturing method of the flexiblecircuit board according to the first embodiment of the presentinvention;

FIGS. 5A to 5F are manufacturing process cross-sectional viewsillustrating an example of a manufacturing method of the flexiblecircuit board according to a second embodiment of the present invention;

FIGS. 6A to 6D are manufacturing process cross-sectional viewsillustrating an example of a manufacturing method of the flexiblecircuit board according to a third embodiment of the present invention;and

FIGS. 7A and 7B are manufacturing process cross-sectional viewscontinued from the processes of FIG. 6.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described with reference tothe accompanying drawings. The drawings are schematic, and it should benoted that proportions between dimensions and the like are not actual.

First Embodiment

A flexible wiring board according to a first embodiment of the presentinvention will be described with reference to FIGS. 1 to 4. In thisembodiment, two examples of a flexible circuit board each on which ahigh-frequency signal transmission wiring is formed will be described.As shown in FIG. 1 and FIGS. 3A and 3B, in a flexible circuit board 10of one example, a plurality of strip lines including signal wirings 12and ground wirings 13 are provided, in the form of a layered conductorpattern, on one main surface of a base film 11 of a thermoset resin. Thesignal wirings 12 are each a two-pair transmission wiring compatiblewith LVDS (Low Voltage Differential Signaling). This flexible circuitboard can be used as a flat cable.

The flexible circuit board has a coverlay film 14 formed of athermoplastic resin. The coverlay film 14 is laminated so as to beadhered to and integrated with the signal wirings 12, ground wirings 13,and base film 11. External terminals 15 for surface mounting of, e.g., asemiconductor element are disposed in a predetermined conductor patternon one main surface of the coverlay film 14 laminated on the base film11.

A first ground layer 16 and a rear side resin film 17 are adhered inthis order below the base film 11 to be integrated. Further, asillustrated in FIG. 3B, the signal wirings 12 and ground wirings 13 areconnected to their corresponding external terminals 15 through acone-shaped first conductive bumps 18 each serving as a conductivemember. The ground wirings 13 are electrically connected to the firstground layer 16 through second conductive bumps 19 each serving as aconductive member. The ground wirings 13 and the first ground layer 16each act as an electromagnetic shield to reduce electromagneticinterference between the signal wirings 12 or signal disturbance causedby external electromagnetic wave.

As illustrated in FIG. 2 and FIG. 3C, in the case of a flexible circuitboard 10 a of another example, a second ground layer 20 and a front sideresin film 21 are sticked to be laminated in a predetermined area on theone main surface of the coverlay film 14. The ground wirings 13 areconnected to the second ground layer 20 through a large number of firstconductive bumps 18 provided along the arrangement direction of eachground wiring 13. Further, the ground wirings 13 are also connected tothe first ground layer 16 through a large number of second conductivebumps 19 provided along the arrangement direction of each ground wiring13. By laminating the front side resin film 21 in this manner, in theflexible wiring board 10 a, higher electromagnetic shielding performancethan in the flexible circuit board 10 can be obtained.

In the abovementioned flexible circuit boards 10 and 10 a, examples ofthe thermoplastic resin suitably used for the coverlay film 14 include aliquid crystal polymer, a thermoplastic polyimide resin, and a compositeresin thereof. The liquid crystal polymer is particularly preferablebecause of its excellent high-frequency transmission characteristics andflexibility. The liquid crystal polymer is multi-axially orientedthermoplastic polymer typified by, e.g., XYDAR (product name of DartcoCo.,) and VECTRA (product name of Clanese Co.,). Other insulating resinmay be added or compounded to the liquid crystal polymer to bedenatured. For example, Vecstar FA type (melting point: 285° C., productname of Kuraray co.), Vecstar CT-X type (melting point: 280° C. to 335°C.), BIAC film (melting point: 335° C.) or the like can be cited.

Examples of the thermoplastic resin further include a polyethyleneterephthalate (PET) resin, a polyether ether ketone (PEEK) resin, and apolyphenylene sulfide (PPS) resin.

In the above flexible circuit boards 10 and 10 a, a thermoset resin usedfor the base film 11 satisfies a condition that the thermal curingtemperature thereof is lower than a glass-transition point Tg or meltingpoint Tm of the abovementioned thermoplastic resin. The thermal curingtemperature of the thermoset resin is the temperature at which theuncured resin film is cured by polymerization or cross-linking. Somethermoplastic resin has the glass-transition point Tg and some does notexhibit a distinct glass-transition point Tg and, in the case of thethermoplastic resin that does not exhibit a distinct glass-transitionpoint Tg, the thermal curing temperature is set lower than the meltingpoint Tm of the thermoplastic resin.

The glass-transition point is generally obtained using a TMA method anda DMA method as a glass transition temperature measurement method(conforming to JIS C6493). In the TMA method, the temperature of a testpiece is raised at a rate of 10° C./minute from a room temperature, athermal analyzer is used to measure the thermal expansion amount of thetest piece in the thickness direction, tangent lines are drawn to acurve around the glass-transition point, and the Tg is calculated fromthe intersection of the tangent lines. In the DMA method (pull method),the temperature of a test piece is raised at a rate of 2° C./minute froma room temperature, a viscoelasticity measuring instrument is used tomeasure the dynamic viscoelasticity and loss tangent of the test piece,and the Tg is calculated from the peak temperature of the loss tangent.In the present invention, the DMA method is used.

Further, the thermoset resin used for the base film 11 satisfies acondition that the elastic modulus of the film is lower than the elasticmodulus of the film of the abovementioned thermoplastic resin. As theelastic modulus, pulling elastic modulus or bending elastic modulus isused. In the present invention, the elastic modulus is measuredaccording to JIS K 7127 or ASTM D 882.

The elastic modulus of the above thermoset resin film can be adjustedadequately by mixing an elastomer with the thermoset resin. Preferableexamples of such an elastomer component include a resin having a lowdielectric tangent, such as a polyester resin, a nitrile rubber (NBR),and a styrene-butadiene rubber (SBR). For example, the pulling elasticmodulus of the thermoset resin film is set in a range of 100 MPa to 1GPa. The pulling elastic modulus of a typical thermoplastic resin filmis in a range from about 2 GPa to 10 GPa; however, by adjusting thepulling elastic modulus of the thermosetting resin film to the abovevalue, the obtained flexible circuit board exhibits sufficient bendingperformance and flexibility.

As the above thermoset resin, a resin that satisfies the above conditionis selected from, e.g., an epoxy resin, a polyester resin, a thermosetpolyimide resin, and a composite resin thereof. Although variouscombinations of the thermoplastic resin and thermoset resin areavailable, it is preferable that a condition is satisfied in which thedielectric constant or dielectric tangent in the film state of thethermoset resin is not higher than the dielectric constant in the filmstate of the thermoplastic resin. By doing this, it is possible toensure extremely excellent transmission characteristics of ahigh-frequency signal in the flexible circuit boards 10 and 10 a.Further, it is preferable that a condition is satisfied in which thethermal expansion coefficient in the film state of the thermoset resinbecomes close to the thermal expansion coefficient of the abovethermoplastic resin. In this manner, surface strain, such as warpage, ofthe flexible circuit board composed of a laminated body of the thermosetresin and thermoplastic resin can be suppressed.

In considering a combination of the thermoplastic resin and thermosetresin, when a liquid crystal polymer is selected as the thermoplasticresin, it is preferable to use a resin composed of oligophenylene etherand styrene-butadiene elastomer as the thermoset resin. This combinationsatisfies the above all conditions. For example, ADFLEMA OPE (productname of Namics Co.,) can be cited.

In the case of the above combination, the dielectric constants of thebase film 11 and the coverlay film 14 are 3 or less, and theelectrostatic tangents of them are 0.003 or less. Thus, the signalwirings 12 in the flexible wiring boards 10 and 10 a exhibit extremelyexcellent transmission/conduction characteristics of a high-speeddigital signal of a band of several GHz to several tens of GHz. Further,appropriate bending performance and flexibility of the flexible circuitboard can be obtained.

The first conductive bump 18 and the second conductive bump 19 are eachformed of a metal material such as silver (Ag), copper (Cu), gold (Au),and solder. The conductor pattern such as the signal wiring, groundwiring, and ground layer is formed of normal Cu. As the conductive bump,one in which metal powders are bonded together with a resin binder so asto allow screen printing to be performed may be used.

A single layer of, e.g., Au, Ag, nickel (Ni) or a composite layer ofNi/Au or Ni/Ag is plated on the metal material surface (e.g., Cumaterial surface) of the external terminal 15.

Various thermoplastic resins or various thermoset resins can be used forthe rear side resin film 17 and front side resin film 21. Further, forexample, a resin like a polyimide film “Kapton” (product name of DuPont-Toray Co.,) can be used.

Next, an example of a manufacturing method of the flexible circuit board10 will be described. FIGS. 4A to 4E are manufacturing processcross-sectional views taken in the Y-direction of FIG. 1. As illustratedin FIG. 4A, a double-sided copper foil clad laminate 24 obtained bycladding a first metal foil 22 and a second metal foil 23 to the frontand rear surfaces of the coverlay film 14 formed of a liquid crystalpolymer having a thickness of about 15 μm to 50 μm is prepared. Thethicknesses of the first and second metal foils 22 and 23 are about 10μm to 20 μm. The first conductive bumps 18 electrically connected to thefirst and second metal foils 22 and 23 are formed at predeterminedpositions of the metal foils. The first conductive bumps 18 are eachformed by a method in which a known metal foil with a bump and a resinfilm are laminated for hot press so as to embed a bump in the resinfilm.

Then, as illustrated in FIG. 4B, the first metal foil 22 is patternedinto a conductor pattern as a strip line by a known etching technique soas to form the signal wirings 12 and ground wirings 13. After that, theconductive bumps 19 each having, e.g., a cone-shape are formed on theground wirings 13 by screen printing.

Then, as illustrated in FIG. 4C, an uncured thermosetting resin film 25having a thickness of about 35 μm to 100 μm, the first ground layer 16,and a one-sided copper foil clad laminate 26 formed of the rear sideresin film 17 having a thickness of about 15 μm to 25 μm are laminatedfrom above, followed by hot press to form the base film 11 in which thesecond conductive bumps 19 connected to the first ground layer 16 areembedded as illustrated in FIG. 4D. The head portion of each cone-shapedconductive bump 19 is crushed flatly to be plastically deformed. Thebase film 11 is thermocompression bonded to the signal wirings 12, theground wirings 13, and the coverlay film 14 to be adhered and integratedwith them.

In the hot press process, atmosphere gas is in, e.g., a decompressedstate, and the heating temperature at that time is a temperature atwhich the uncured thermosetting resin film 25 is thermally cured. Thistemperature is lower than the glass-transition point or melting point ofthe coverlay film 14 formed of the thermoplastic resin and is set to,e.g., about 180° C. to 230° C. The pressurization is carried out at,e.g., 10 kgf/cm² to 40 kgf/cm². Thus, hot press can be carried out witha small force, so that the flexible circuit board 10 is less subject towarpage.

In FIG. 4E illustrating a state where the front and rear surfacesillustrated in FIG. 4D are turned upside down, the second metal foil 23are patterned into a conductor pattern to form the external terminals15. Further, a plated layer (not illustrated) is formed on each of theexternal terminals 15. In this manner, the flexible wring board 10having the cross-section illustrated in FIG. 3B is obtained.

In the manufacturing process of the flexible circuit board 10, a largestandard size structure is obtained by the lamination and integration,followed by cutting into predetermined shapes whereby a plurality of theflexible circuit boards 10 are obtained. Preferably, the formation ofplated layer on the surface of each external terminal 15 is performed ina state before cutting the large standard size structure with theconductor-patterned signal wirings 12 and ground wirings 13 used aspower feeding layers.

In the manufacturing process of the flexible circuit board 10, theone-sided copper foil clad laminate 26 may be one obtained by cladding ametal foil or applying electrolytic plating to the surface of a resinfilm.

A manufacturing process of the flexible wiring plate 10 a issubstantially the same as that of the flexible circuit board 10. In thiscase, when the second metal foil 23 is patterned into a conductorpattern, the second ground layer 20 is formed together with the externalterminals 15. Then, the front side resin film 21 having a thickness of15 μm to 25 μm and coating the second ground layer 20 is adhered to thethermoplastic resin sheet or thermoset resin sheet by, e.g., a thermalroll laminating method.

In the flexible circuit boards 10 and 10 a according to the presentembodiment, the coverlay film 14 (first insulating layer) formed of thethermoplastic resin is adhered to and integrated with the base film 11(second insulating layer) formed of the thermoset resin. In thisconnecting process, the base film 11 is thermocompression bonded to thecoverlay film 14 and the like at a curing temperature lower than theglass-transition point or melting point of the coverlay film 14.Therefore, in the multilayering of the flexible circuit board accordingto the present embodiment, softening or thermal flow of the coverlayfilm 14 caused by hot press is suppressed. This prevents occurrence ofposition gap in the signal wirings 12 and ground wirings 13 which areformed into a conductor pattern on the main surface of the coverlay film14 and the first and second conductive bumps 18 and 19 which areconductive members. In this manner, the multilayering of the flexiblecircuit board having the interlayer dielectric layer formed of thethermoplastic resin can be facilitated.

Further, in the connecting process between the base film 11 and thecoverlay film 14 having the conductor pattern and conductive members, itis not necessary to provide an adhesive layer that may cause thedielectric tangent to increase by using adhesive. Thus, a reduction inthe dielectric constants of the base film 11 and coverlay film 14 and areduction in the dielectric tangents of them can be achieved easily. Asa result, speed-up of a high-frequency signal and reduction in thedielectric loss can be easily achieved in the flexible circuit board,whereby a high-performance flexible circuit board having excellenttransmission characteristics can be obtained.

Further, in the flexible circuit boards 10 and 10 a according to thepresent embodiment, no adhesive is required in their lamination andintegration processes, thereby simplifying the manufacturing process ofthe flexible circuit board. As described above, even in thehigh-performance flexible circuit board, increase in productivity can beattained easily, resulting in a reduction in the manufacturing cost andcost of the flexible circuit board itself.

Second Embodiment

A flexible circuit board according to a second embodiment will bedescribed with reference to FIG. 5 illustrating an example of amanufacturing process thereof. In the second embodiment, a flexiblecircuit board provided with two-layer circuit wirings will be described.

A one-sided copper foil clad laminate having a metal foil cladded on onemain surface of a thermoplastic resin film like a liquid crystal polymerhaving a thickness of about 25 μm to 50 μm is prepared. Then, asillustrated in FIG. 5A, the metal foil is selectively etched to form aconductor pattern, whereby a first circuit pattern 32 is formed on onemain surface of a first insulating layer 31 formed of a thermoplasticresin film.

Subsequently, as illustrated in FIG. 5B, conductive bumps 33 are formedon predetermined positions of the first circuit pattern 32 as in thefirst embodiment. Then, as illustrated in FIG. 5C, an uncuredthermosetting resin film 34 and a metal foil 35 are laminated fromabove, followed by hot press. Then, as illustrated in FIG. 5D, the headportions of the conductive bumps 33 are crushed flatly to be plasticallydeformed, whereby a second insulating layer 36 in which the conductivebumps 33 connected to the metal foil 35 are embedded is formed. Thesecond insulating layer 36 is formed of a thermally cured resin filmobtained as a result of thermal curing applied to the uncuredthermosetting resin film 34 and is thermocompression bonded and adheredto the first insulating layer 31 and the first circuit pattern 32. Asdescribed in the first embodiment, in the hot press, the heatingtemperature at that time is a temperature at which the uncuredthermosetting resin film 34 is thermally cured and is set lower than theglass-transition point or melting point of the first insulating layer31.

Subsequently, as illustrated in FIG. 5E, the metal foil 35 is patternedinto a conductor pattern so as to form a second circuit pattern 37.Then, as illustrated in FIG. 5F, a solder resist 38 exposingpredetermined areas of the second circuit pattern 37 is formed.

In the manner as described above, a flexible circuit board having aprinted circuit in which the first insulating layer 31 of thethermoplastic resin film and the first circuit pattern 32, the secondinsulating layer 36 of the thermosetting resin film and the secondcircuit pattern 37 are adhered to be integrated is obtained.

The thermoplastic resin and thermoset resin used in the presentembodiment are selected from among those described in the firstembodiment. Also in the present embodiment, it is preferable to use acombination of a liquid crystal polymer selected as the thermoplasticresin and a resin composed of oligophenylene ether and styrene-butadieneelastomer selected as the thermoset resin.

The second embodiment can provide the same effects as those of the firstembodiment, and there can be provided a flexible circuit board having awiring circuit compatible with high frequency (up to GHz) of, e.g., acomputer CPU clock.

Third Embodiment

Next, a flexible circuit board according to a third embodiment will bedescribed with reference to FIGS. 6 and 7 illustrating an example of amanufacturing process thereof. In the third embodiment, a flexiblecircuit board provided with three or more-layer circuit wirings will bedescribed.

For example, metal foils cladded on the both sides of a double-sidedcopper foil clad laminate as exemplified in the first embodiment arepatterned into conductor patterns to prepare a double-sided circuitboard 41 as illustrated in FIG. 6A. In the double-sided circuit board41, circuit patterns 43 are formed on both sides of a thermoplasticresin film 42 formed of a liquid crystal polymer. The front-side circuitpattern 43 and rear-side circuit pattern 43 are connected to each otherat predetermined positions by first conductive bumps 44. Thethermoplastic resin film 42 serves as a first dielectric layer.

Subsequently, as illustrated in FIG. 6B, cone-shaped second conductivebumps 45 are formed on the front-side circuit pattern 43 by screenprinting in the same manner as that of the first embodiment. Then, asillustrated in FIG. 6C, an uncured thermosetting resin film 46 and asheet-like supporting member 47 made of a resin or the like arelaminated from above onto and integrated with the double-sided circuitboard 41 having the second conductive bumps 45 by hot process. Theheating temperature in the hot press is a temperature at which theuncured thermosetting resin film 46 is thermally cured and is set lowerthan the glass-transition point or melting point of the thermoplasticresin film 42. The pressurization is carried out at, e.g., 10 kgf/cm² to40 kgf/cm². Thus, hot press can be carried out with a small force, sothat the flexible circuit board 10 is less subject to warpage.

Then, as illustrated in FIG. 6D, the sheet-like supporting member 47 ispeeled off to form a stacked circuit board 48. In this stacked circuitboard 48, the thermoplastic resin film 42 and thermosetting resin film46 are thermocompression bonded to each other, and circuit patterns 43using the thermoplastic resin film 42 and thermosetting resin film 46 astheir interlayer dielectric layers are formed. Further, the two-layercircuit patterns 43 are appropriately connected to each other by thefirst conductive bumps 44, and the second conductive bumps 43 areprovided at predetermined positions of the circuit pattern 43. Thethermosetting resin film 46 thermocompression bonded to thethermoplastic resin film 42 serves as a second insulating layer.

The thermoplastic resin serving as the thermoplastic resin film 42 andthermoset resin serving as the thermosetting resin film 46 used in thepresent embodiment are selected appropriately from among those describedin the first embodiment. Also in the third embodiment, it is preferableto use a combination of a liquid crystal polymer selected as thethermoplastic resin and a resin composed of oligophenylene ether andstyrene-butadiene elastomer selected as the thermoset resin.

A flexible circuit board having a required number of wiring layers ismanufactured by a build-up method using the abovementioned double-sidedcircuit board 41 and stacked circuit board 48. For example, asillustrated in FIG. 7A, one double-sided circuit board 41 and a requirednumber of the stacked circuit boards 48 are set up. In this set-upprocess, the above circuit boards are set to a predetermined positionfollowed by lamination. In this case, positions of the double-sidedcircuit board 41 or stacked circuit board 48 needs to be adjusted suchthat the circuit pattern 43 is connected to the second conductive bumps45. The circuit pattern 43 of the double-sided circuit board 41 may beformed into a required pattern different from the pattern illustrated inFIG. 6A. Similarly, in the plurality of stacked circuit boards 48, eachof the circuit patterns 43 may be formed into a required pattern.

Multilayer lamination press is applied to the setup double-sided circuitboard 41 and stacked circuit boards 48 by hot press of a predeterminedtemperature/pressure. In this manner, a multilayered flexible printedcircuit board as illustrated in FIG. 7B is obtained. The heatingtemperature in the hot press is a temperature at which the thermoplasticresin film 42 is soften to be thermocompression bonded to thethermosetting resin film 46. The pressurization is carried out at, e.g.,10 kgf/cm² to 70 kgf/cm². Thus, hot press can be carried out with asmall force, so that the flexible circuit board 10 is less subject towarpage. In this hot press, the thermosetting resin film 46 is cured bypolymerization or cross-linking, so that the plastic deformation due toheating does not occur.

In addition to the above method, there are available variousmanufacturing method of the flexible circuit board having a multilayerprinted circuit. For example, in place of the thermal roll laminatingmethod of the thermosetting resin film 46 illustrated in FIG. 6C, aone-sided copper foil clad laminate having a metal foil cladded on onemain surface of the uncured thermosetting resin film 46 is prepared.Subsequently, hot press is applied with the uncured thermosetting resinfilm 46 facing down so as to thermocompression bond the uncuredthermosetting resin film 46 to the double-sided circuit board 41 in thestate of FIG. 6B for lamination and integration. The hot press in thiscase is performed under the same condition as that described in thefirst embodiment. Then, the metal foil is patterned into a conductorpattern. Thus, resulting from one-layer build-up, a flexible circuitboard having three-layer circuit patterns is obtained.

Further, the uncured thermosetting resin film 46 and a metal foil havingconductive bumps at its predetermined areas are hot pressed onto thedouble-sided circuit board 41 in which build-up of one layer has beencompleted from below for lamination and integration. In this manner, aflexible circuit board having a printed circuit with a four-layercircuit patterns is obtained.

The third embodiment can provide the same effects as those of the firstembodiment. Further, in a three-layer or more multilayer structure ofthe flexible circuit board, occurrence of the position gap in thecircuit pattern and conductive bumps can be easily suppressed. Thisallows the positioning accuracy in the lamination of a plurality ofresin films to be improved to easily achieve miniaturization of thecircuit pattern and the like. In this manner, a high-performanceflexible circuit board in which the high-density wiring and short,small, light, and thin design have been achieved is simplified in itsmanufacturing process, eliminating the need for high-level technicalmanagement and high-performance manufacturing facility. As a result, acost reduction of the high-performance flexible circuit board can beachieved.

Although preferred embodiments of the present invention have thus beendescribed, the present invention is by no means limited to theembodiments described above. Various modifications and changes may bemade in a concrete embodiment by those skilled in the art withoutdeparting from the technical concept and technical scope of theinvention.

As a substitute for the conductive bump as described in the aboveembodiments, a plated via formed between circuit layers used in amultilayer lay-up process may be used.

1. A flexible circuit board comprising: a first insulating layer formedof a thermoplastic resin having a conductive layer on its one mainsurface; and a second insulating layer formed of a thermoset resinhaving a conductive layer on its one main surface and adhered to andintegrated with the first insulating layer, wherein the curingtemperature of the thermoset resin is lower than the glass-transitionpoint or melting point of the thermoplastic resin, and the elasticmodulus of the second insulating layer formed of the thermoset resin islower than the elastic modulus of the first insulating layer formed ofthe thermoplastic resin.
 2. The flexible circuit board according toclaim 1, wherein the thermoplastic resin is a liquid crystal polymer,and the thermoset resin is a resin obtained by mixing oligophenyleneether and styrene-butadiene elastomer.
 3. The flexible circuit boardaccording to claim 1, wherein the first and second insulating layers arealternately laminated to form a multilayer structure.
 4. The flexiblecircuit board according to claim 2, wherein the first and secondinsulating layers are alternately laminated to form a multilayerstructure.
 5. The flexible circuit board according to claim 1, whereinthe conductive layer formed on the one main surface of the secondinsulating layer includes a signal circuit pattern and a ground circuitpattern, the second insulating layer further has a first ground layer tobe adhered to the other main surface thereof, and the first ground layeris electrically connected to the ground circuit pattern through thesecond insulating layer.
 6. The flexible circuit board according toclaim 5, wherein the first insulating layer has a second ground layer tobe adhered to the one main surface thereof, and the second ground layeris electrically connected to the ground circuit pattern of the secondinsulating layer through the first insulating layer.